Field of the Invention
The present invention relates to a planar antenna having a matching circuit patterned on a printed circuit board, and a radar apparatus having array antennas using a plurality of such planar antennas.
Background Information
Conventionally, a three-dimensional imaging radar apparatus is used as a method of inspecting the inside of a structure made of a material (non-metal), such as concrete or wood, in a non-destructive manner. The three-dimensional imaging radar apparatus performs three-dimensional imaging of the inside of the structure by performing a migration process on data measured by a plurality of receiving antennas. Detectable targets include gaps, cavities, metallic matter, regions with different conductivities or permittivities, etc., in the structure. The planar resolution of an image of such a radar apparatus depends on the wavelength of an electromagnetic wave to be used, and the vertical resolution of an image depends on the width of the frequency band of an electromagnetic wave to be used. Hence, to obtain a high-definition, three-dimensional image, it is possible to raise the frequency of an electromagnetic wave used by the radar and to further widen the frequency band of the electromagnetic wave. Currently, the frequency band of the three-dimensional imaging radar apparatus is 100 MHz to 4.5 GHz. Therefore, it is possible to further raise the upper limit of frequencies at which the current three-dimensional imaging radar apparatus operates, and to further widen the frequency band where an electromagnetic wave operates.
In the three-dimensional imaging radar apparatus, a bow-tie shaped planar antenna is effectively used in terms of that the antenna can be made relatively small in size and can achieve a broadband. As such a bow-tie shaped antenna, a planar antenna using a plurality of self-complementary antenna elements is used. As an example of the planar antenna, a planar antenna with a very small reflection loss in an extremely broadband range is known as described in JP 2005-130292 A.
The above-described planar antenna includes a dielectric substrate; a plurality of antenna conductors which are formed on one side of the dielectric substrate and which are pseudo-self-complementary on this side; and a plurality of feeding conductors symmetric with respect to a symmetric plane of the antenna conductors. At this time, a space of 1/10 or less of the wavelength in a vacuum of a used frequency is provided at the center of a rotational symmetry between the plurality of antenna conductors.